Backport changes to lifecycle.mdx from 2025-06-18 to draft. (modelcontextprotocol#997)

* Backport changes to `lifecycle.mdx` from `2025-06-18` to `draft`.

In the PR releasing a new revision of the spec (modelcontextprotocol#783), commit e547813
changed a `SHOULD` to `MUST` in Lifecycle Operation section, but the
change was not backported to the draft file.

This PR backports the change avoiding potential regressions.

* Update docs/specification/draft/changelog.mdx

---------

Co-authored-by: Jonathan Hefner <[email protected]>

Author: blkt

docs/specification/draft/basic/lifecycle.mdx

@@ -174,7 +174,7 @@ Capability objects can describe sub-capabilities like:
 During the operation phase, the client and server exchange messages according to the
 negotiated capabilities.
 
-Both parties **SHOULD**:
+Both parties **MUST**:
 
 - Respect the negotiated protocol version
 - Only use capabilities that were successfully negotiated